In case anybody is trying to figure out what the "other factors" are, here is a few additional data points:
1. Facebook suggested my next-door neighbor to me as a potential friend.
2. I have fewer than 20 Facebook friends, and none of them are my neighbors, most are family and relatives in other cities.
3. My Facebook account is linked to a unique email address which I created only for Facebook, and which has not been revealed to any other person or website.
4. I have never imported any contacts into Facebook from any other source.
5. I do not have the Facebook app installed on my phone, and I have never connected to Facebook from my phone using the browser.
6. I have virtually no other apps installed on my phone except for Google Mail, Google Calendar, and Google Maps.
7. No other apps on my phone have been given access to location data, and the GPS is always disabled (to save battery) except when I am actively using Google Maps somewhere I am unfamiliar, (so never anywhere near my own neighborhood).
8. I provide an open wi-fi access point (no password), which is easily accessible from my next-door neighbor's house.
9. I have logged into Facebook using a computer connected to that open wi-fi network.
Could the "other factors" be as simple as IP address?